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.\"     @(#)ioctl.2	8.2 (Berkeley) 12/11/93
.\"
.Dd December 19, 2010
.Dt IOCTL 2
.Os
.Sh NAME
.Nm ioctl
.Nd control device
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/ioctl.h
.Ft int
.Fn ioctl "int d" "unsigned long request" "..."
.Sh DESCRIPTION
The
.Fn ioctl
function manipulates the underlying device parameters of special files.
In particular, many operating
characteristics of character special files (e.g. terminals)
may be controlled with
.Fn ioctl
requests.
The argument
.Fa d
must be an open file descriptor.
.Pp
An
.Fn ioctl
.Fa request
has encoded in it whether the argument is an
.Dq in ,
.Dq out ,
or
.Dq inout
parameter, and the size of the first variadic argument
in bytes.
Note that there can be only one variadic argument but cannot be represented as
a
.Ft "void *"
argument in the prototype because this would require a cast to pass integral
types without warnings.
Macros and defines used in specifying an
.Fn ioctl
.Fa request
are located in the header
.In sys/ioctl.h .
.Sh GENERIC IOCTLS
Some ioctls are applicable to any file descriptor.
These include:
.Bl -tag -width "xxxxxx"
.It Dv FIOCLEX
Set close-on-exec flag.
The file will be closed when
.Xr exec 3
is invoked
(This is equivalent to
.Fn fcntl
.Dv F_SETFD
.Dv FD_CLOEXEC
and the
.Fn fcntl
form should be preferred).
.It Dv FIONCLEX
Clear close-on-exec flag.
The file will remain open across
.Xr exec 3
(This is equivalent to
.Fn fcntl
.Dv F_SETFD
.Dv 0
and the
.Fn fcntl
form should be preferred).
.El
.Pp
Some generic ioctls are not implemented for all types of file
descriptors.
These include:
.Bl -tag -width "xxxxxx"
.It Dv FIONREAD "int"
Get the number of bytes that are immediately available for reading.
.It Dv FIONWRITE "int"
Get the number of bytes in the descriptor's send queue.
These bytes are data which has been written to the descriptor but
which are being held by the kernel for further processing.
The nature of the required processing depends on the underlying device.
For tty devices, these bytes are typically queued for delivery
to the tty hardware.
For TCP sockets, these bytes have not yet been acknowledged by the
other side of the connection.
For files, this operation always returns zero as files do not have
send queues.
.It Dv FIONSPACE "int"
Get the free space in the descriptor's send queue.
This value is the size of the send queue minus the number of bytes
being held in the queue.
Note: while this value represents the number of bytes that may be
added to the queue, other resource limitations may cause a write
not larger than the send queue's space to be blocked.
One such limitation would be a lack of network buffers for a write
to a network connection.
.It Dv FIONBIO "int"
Set non-blocking I/O mode if the argument is non-zero.
In non-blocking mode,
.Xr read 2
or
.Xr write 2
calls return
.Dv \-1
and set
.Va errno
to
.Er EAGAIN
immediately when no data is available
(This is equivalent to
.Fn fcntl
.Dv F_SETFL
.Dv O_NONBLOCK
and the
.Fn fcntl
form should be preferred).
.It Dv FIOASYNC "int"
Set asynchronous I/O mode if the argument is non-zero
(This is equivalent to
.Fn fcntl
.Dv F_SETFL
.Dv O_ASYNC
and the
.Fn fcntl
form should be preferred).
In asynchronous mode, the process or process group specified by
.Dv FIOSETOWN
will start receiving
.Dv SIGIO
signals when data is available.
The
.Dv SIGIO
signal will be delivered when data is available on the file
descriptor.
.It Dv FIOSETOWN, FIOGETOWN "int"
Set/get the process or the process group (if negative) that should receive
.Dv SIGIO
signals when data is available
(This is equivalent to
.Fn fcntl
.Dv F_SETOWN
.Ft pid_t
and the
.Ft fcntl
form should be preferred).
.El
.Sh RETURN VALUES
If an error has occurred, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn ioctl
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa d
is not a valid descriptor.
.It Bq Er EFAULT
.Fa argp
points outside the process's allocated address space.
.It Bq Er EINVAL
.Fa request
or
.Fa argp
is not valid.
.It Bq Er ENOTTY
.Fa d
is not associated with a character
special device; or
the specified request does not apply to the kind
of object that the descriptor
.Fa d
references.
.El
.Sh SEE ALSO
.Xr mt 1 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr intro 4 ,
.Xr tty 4
.Sh HISTORY
An
.Fn ioctl
function call appeared in
.At v7 .
